MOSCOW, April 22 (Reuters) - Russian opposition leader Alexei Navalny was convicted of libel on Tuesday in a ruling his lawyer said could potentially lead to the jailing of the anti-corruption blogger who is one of President Vladimir Putin's most prominent critics.

The verdict added to the pressure on Navalny, who is serving a suspended five-year sentence on a theft conviction he says was orchestrated by the Kremlin and faces trial on a separate theft charge later this week.

"This is clearly just another attempt to chase me into a corner," Navalny, who has been under house arrest since March, said as he left court.

The judge found him guilty of libel over a Twitter posting calling a city lawmaker from the ruling United Russia party a drug addict, and fined him 300,000 roubles ($8,400).

Navalny said he would appeal. He had denied guilt, pointing out that he is barred from posting on social media under the terms of his house arrest, though relatives and colleagues do post on his accounts.

Libel is not punishable by prison time. But a lawyer for Navalny, Vadim Kobzev, said the guilty verdict would open the door to a judge putting Navalny behind bars during the upcoming trial instead of under house arrest.

It could also enable prison authorities to ask a court to remove the suspension of his theft sentence and jail him.

Navalny, 37, gained prominence with an Internet-based crusade against alleged official corruption and helped lead a wave of street protests against Putin in 2011-2012.

He said on Tuesday that he would continue his anti-corruption activity.

"If somebody thinks ... we will stop publishing investigations, it's not going to happen," he said.

The protests were the biggest of Putin's 14-year rule but failed to prevent him winning a third Kremlin term in March 2012. Critics say the legal pressure on Navalny is part of a renewed clampdown on dissent.

Navalny's house arrest keeps him away from street protests, and Russian internet providers blocked access to his blog last month at the order of federal prosecutors.

Jailing him would anger Kremlin opponents and would be certain to add to Western criticism of Putin at a time when he is embroiled in a tense showdown with the United States and European Union over the future of neighbouring Ukraine.